The plasma mass spectrometer is known as an analyzer for the highly sensitive analysis of inorganic elements. By means of this instrument, a sample to be analyzed that has been nebulized, converted to micro particles, etc. is introduced into plasma formed on a plasma torch, the elements contained in the sample are ionized, and then the ions in the plasma are extracted in the form of an ion beam and a mass spectrometric analysis of the sample is performed by detecting those ions. The plasma into which the sample is introduced is either an inductively coupled plasma (ICP) that is generated using as the energy source a high-frequency electromagnetic field provided from a coil near the plasma torch, or a microwave plasma produced by microwaves introduced into the tip of the plasma torch.
This instrument comprises an interface for sampling and then skimming a portion of the generated plasma. Usually this interface comprises two cone parts, a sampling cone and a skimmer cone. These cone parts have circular cone-shaped projections that face the plasma torch side, and there is a small orifice at the tip of these projections. A portion of the plasma formed on the plasma torch is sampled and then skimmed while passing through these small orifices and reaches the back side of the skimmer cone, which is disposed on the downstream side.
The ions present in the skimmed plasma are provided in the form of an ion beam by extraction electrodes positioned in the front part of an ion optical system. The extraction electrodes include an electrode set at negative potential and extract the positive ions in the plasma with the electric field formed by that electrode.
The extracted ions further pass through the ion optical system, that typically includes an ion deflection lens and an ion guide, and are introduced into the ion separation part behind the ion optical system. By means of the ion separation part, ions are selected and separated based on their mass-to-charge ratio such that only specific ions reach the detector behind the ion separation part. The ion separation part typically has a multi-electrode structure, such as a quadrupole.
In order to improve the analysis precision of this plasma mass spectrometer, there is a demand for the removal of the ions (interference ions) that interfere with other specific ions during mass spectrometric analysis. These interference ions are typically polyatomic ions that comprise multiple atoms including the element of the carrier gas.
This problem can be solved by inducing a collision/reaction effect with gas that is additionally introduced before the ions reach the ion separation part (JP (Kohyo) 2005-535071, JP (Kohyo) 2005-519450, JP (Kohyo) 11-509036). Since carrier gas that forms the primary component of the plasma is typically argon gas, the interference ions are polyatomic ions that contain argon atoms. These polyatomic ions are removed or decomposed and isolated from the ion beam by deceleration or a reaction such as charge transfer as a result of colliding with the molecules of the additional gas.
There are a variety of positions for the introduction of additional gas, such as inside the cone parts that form the interface (JP (Kohyo) 2005-535071), directly behind the interface (JP (Kohyo) 2005-519450 particularly the example in FIG. 4], JP (Kohyo) 11-509036), and inside the components that form the ion optical system (JP (Kohyo) 11-509036). The additional gas is typically hydrogen gas, helium gas, ammonia, argon, a mixed gas of several of these gases, and similar gases.
A second method for solving this problem is the method whereby the reduction of polyatomic ions is promoted by forming a region of relatively low vacuum, that is, relatively high pressure, during the course of skimming the plasma so as to cause the polyatomic ions to collide with the gas molecules in this region (JP (Kohyo) 2005-519450 [particularly the examples in FIGS. 2 and 3], JP (Kokai) [Unexamined Patent Publication] 10-40857). This region can have a portion having a relative small capacity inside the orifices in the cone parts forming the interface (JP (Kokai) [Unexamined Patent Publication] 10-40857) and a portion having a relatively large capacity directly behind the skimmer cone forming the interface (JP (Kokai) [Unexamined Patent Publication] 10-40857).